1. Field of the Invention
The present invention relates to an optical waveguide element or device for separating or demultiplexing a wavelength-multiplexed input light signal for every wavelength and outputting the same therefrom. The present invention relates particularly to an optical multiplexing/demultiplexing device having a waveguide formed within a planar waveguide.
2. Description of the Related Art
In the field of optical communications, a wavelength division multiplexing (WDM) system has been developed which brings a plurality of signals into a signal form that is set as different lights and transmits them via an optical fiber. The present system needs to multiplex or demultiplex the lights that are different in wavelength for their input/output. Various types of elements or devices such as an array waveguide grating device, a device using a grating, etc. have heretofore been known as such types of optical branching or demultiplexing devices. FIGS. 1 and 2 show examples of optical demultiplexing devices each using a grating.
The device shown in FIG. 1 has a structure in which a linear optical waveguide 2 is provided on a substrate 1 and a linear chirp grating 3 is formed thereon. A planar waveguide 4 is provided side by side with the linear optical waveguide 2. The planar waveguide 4 takes a configuration in which optical waveguides 5 to output lights are connected thereto at its boundary surface. Light incident from one end of the linear optical waveguide 2 is reflected by the grating 3 and input to the planar waveguide 4 through the linear optical waveguide 2.
The cycle of the grating becomes small as it proceeds to its end. Thus, the light propagates so as to converge on the boundary portion of the planar waveguide 4 as shown in FIG. 1. Light-gathering points differ from one another for every wavelength, depending on the state of the interference of light, which is in turn dependent on the wavelengths at this boundary. The provision of the optical waveguides 5 at the boundary makes it possible to output the lights for every wavelength. A method of avoiding the use of such a special grating as seen in the structure of FIG. 1 has also been proposed as shown in FIG. 2.
In the structure of FIG. 2, an input waveguide 12 and a curved waveguide 13, laid out in an arc form other than the linear waveguide, is provided with equidistant gratings on a substrate 11. Further, a planar waveguide 14 having a shape extending along the curved waveguide 13 is provided, and output waveguides 15 are placed in a central position of a circular arc of the curved waveguide 13. The gratings are set diagonally to the center of the waveguide in such a manner that lights reflected by the gratings converge on the center of the circular arc. If the structure of FIG. 2 is adopted, then the lights can be focused on one point even if equidistant gratings are provided.
However, the conventional structure has a drawback in that the optimum focal position is substantially one point, i.e., an output value decreases in the case where each wavelength deviates from the focal point. A problem arises in that the acquisition of a certain degree of output by wavelengths which deviate from the optimum focal position results in the need to reduce a change in the focal position with respect to the distance extending from each grating to the focal point, thus leading to an increase in the overall length of the device.
The present invention aims to replace a conventionally used grating with a reflecting surface producible according to a waveguide producing process having a structure comprising a curved waveguide for input, output waveguides and a planar waveguide that are each provided within a substrate. The curved waveguide for input is discontinuous, and the curved waveguide and planar waveguide are spaced away from each other with an equal interval interposed therebetween.
Further, a light signal is reflected by each of the individual, discontinuous surfaces and is wavelength-demultiplexed through the substrate and planar waveguide. The demultiplexed lights are respectively focused on the output waveguides every wavelength. As a result, a structure can be formed which includes an optimum waveguide shape and reflecting surfaces, and hence a device that has improved controllability can be formed.